He's on to something - Pandemic Influenza is still top of the risks (though as a caveat, this document is due an update this year. CCS at the Cabinet Office have a few other things on their plate...)My son explained his theory on something else he thinks Mother Nature herself has been trying to implement to reduce population growth and what us, the human race, is doing to beat these plans.
In at least 2 instances, Aids and Ebola, nature has introduced a new natural killer intended to wipe out huge numbers of humans, we have managed to either beat or mitigate by science and/or medicine these two. He believes more such diseases will appear over the next few years and eventually one of them will get the job done.
I think it's a strange theory but I do wonder if perhaps there is a grain of truth in it. I've heard ex military friends saying a really big killing war is what we need to aid population control so perhaps my son's theory isn't as daft as it sounds and it's less extreme than a world war.